Remote control system with plural modulation



Aug. 4, 1970 w. s. REYNOLDS 3 522,536

REMOTE CONTROL SYSTEM WITH PLURAL MODULATION Filed NOV. 24. 1967 3Sheets-Sheet l 3 r T [24 O 1' Wi/lara' s. Reynolds INVENTOR.

Aug. 4, 1970 w. s. REYNOLDS 3,522,536 REMOTE'CONTROL SYSTEM WITH PLURALMODULATION Filed Nov. 24, 1967 3 Sheets-Sheet 2 w mom Willard S.Reynolds INVENTOR. 40%;.

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United States Patent Ofice 3,522,536 Patented Aug. 4, 1970 3,522,536REMOTE CONTROL SYSTEM WITH PLURAL MODULATION Willard S. Reynolds,Hollywood, Fla., assignor to Telectron Company, Division of Elliott &Evans, Inc., a corporation of Ohio Filed Nov. 24, 1967, Ser. No. 685,583Int. Cl. H04b 7/00 US. Cl. 325 -37 18 Claims ABSTRACT OF THE DISCLOSUREA remote control system in which an RF oscillator transmits a carrierfrequency output sequentially modulated by two different audio frequencytones derived from two audio oscillators energized simultaneously withthe RF oscillator. An oscillating output from one of the audiooscillators applies a reverse bias to the transistor associated with theother audio oscillator to prevent simultaneous operation. One of theaudio oscillators has a time controlled operational period in order toobtain sequential modulation at the two different audio frequencies.

This invention relates to a remote radio control system for controllingpower-operated devices such as commercial doors.

One of the important considerations in providing a remote radio controlsystem is to prevent unauthorized or unintentional operation of thecontrol mechanism. To provide such safeguards however, usually involvesexpensive modification of both the transmitter and receiver componentsassociated with the remote control system.

A further consideration in controlling power-operated mechanism fromsome remote location by radio signals is to accommodate severaldifferent operational controls. In connection with commercial doors, forexample, it is necessary to reverse and stop operation of power-operatedmotors for correspondingly moving or stopping movement of the door.Accordingly, both the transmitter and receiver components of the remotecontrol system must have such multifunction capabilities.

It is therefore an important object of the present invention to providea remote control system in which the transmitter component avoidsexpensive complexities but is nevertheless capable of performingmultifunctions and produce a coded output keyed to the receivercomponent. Similarly, the receiver component must have the multifunctioncapability and respond exclusively to the coded output of thetransmitter component.

In accordance with the present invention, the transmitter componentfeatures a solid state, radio frequency oscillator immediately triggeredinto operation upon closing of a switch to supply DC operating voltagefor establishing a suitable base bias for the transistor associated withthe radio frequency oscillator. Bias voltage is also simultaneouslyapplied from the voltage source to the transistors associated with apair of audio oscillators connected in parallel to the radio frequencyoscillator for modulating the carrier frequency output thereof. A timingcircuit is associated with one of the audio oscillators for eitherdelaying operation thereof or limiting its operation to a relativelyshort period. Thus, during the non-operatmg period of the audiooscillator controlled by the timing circuit, the other audio oscillatoris operative to modulate the output of the radio frequency oscillator atone audio frequency. When the time controlled oscillator begins tooperate, at another audio frequency, a reverse bias is applied to theother audio oscillator in order to prevent simultaneous operation ofboth oscillators. Accordingly, the output of the radio frequencyoscillator is sequentially modulated at two dilferent audio frequencies.The carrier frequency of the radio frequency oscillator, the timedoperational period of one of the audio oscillators and the respectiveaudio frequencies of the audio oscillators therefore provide a largenumber of code factor combinations that may be utilized to insureprivacy of the radio control system.

In one form of the invention, switch means are provided to change theoperational mode of the transmitter. Three operational modes are thuspossible consisting of sequential modulation of the carrier frequencyoutput at the two audio frequencies, exclusive modulation of the carrierfrequency output by one of the audio frequencies and exclusivemodulation of the carrier frequency output by the other of the audiofrequencies. The transmitter component istherefore arranged to recognizenot only the code factors or the values of the carrier and audiofrequencies associated with the transmitter component but also iscapable of distinguishing between the three different operational modesin order to perform diverse functions such as effecting upward movement,downward movement or stopping movement of a commercial door.

These together with other objects and advantages which will becomesubsequently apparent reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part hereof, whereinlike numerals refer to like parts throughout, and in which:

FIG. 1 is an'electrical circuit diagram illustrating one form oftransmitter in accordance with the present invention.

FIG. 2 is an electrical circuit diagram illustrating another form oftransmitter.

FIG. 3 is an electrical circuit diagram illustrating a third form oftransmitter.

FIGS. 4 and 5 are graphical illustrations showing the signal wave formsassociated with the two audio frequency modulating signals.

FIG. 6 is an electrical circuit diagram illustrating the receivercomponent associated with the transmitter illustrated in FIG. 3.

Referring now to the drawings in detail, FIG. 1 illustrates one form oftransmitter component generally referred to by reference numeral 10.Signal energy is radiated from the transmitter component by means of aradio frequency oscillator 12 having a transistor 14 of the NPN type.The collector of transistor 14 is connected to an oscillating outputcircuit having an inductive loop 16 connected in parallel with a tuningcapacitor I frequency range. The output circuit is connected by means ofthe feedback capacitor to the base of transistor 14 in order tointroduce into the base of sulficient voltage of proper phase so thatthe AC component of the collector current will persist for sustainedoscillation of the output circuit at the carrier frequency. Toward thisend, proper base bias is derived from a DC source of voltage such asbattery 22 upon closing of the switch 24 grounding the negative terminalof the battery. The positive terminal of the battery is connectedthrough resistors 26 and 28 to the base of transistor 14 while a leakageresistor 30 is connected between ground and the input juncture 32between the resistors 26 and 28 in order to maintain the proper basebias. As will be hereafter explained, modulating signal voltage may alsobe applied to the base of transistor '14 through the signal couplingcapacitor 34 connected to the input juncture 32. Also in order toestablish proper bias for the output circuit, the inductive loop 16 isconnected to the DC voltage source 22 through a choke coil 36 whichprevents AC current from being conducted through the voltage source andthe base of the transistor. Emitter bias for the transistor 14 isestablished by means of the emitter resistor 40 connected between groundand the emitter through the choke coil 38 preventing any AC componentsfrom being conducted through ground to the voltage source.

One of the modulating signals applied to the radio fre quency oscillatorthrough coupling capacitor 34, is de rived from the audio oscillator 42which includes an N'PN type of transistor 44. An emitter resistor 46 isconnected between ground and the emitter of transistor 44 while theoutput collector of the transistor is connected by the output line 48 tothe signal coupling capacitor 34 for supplying a modulating signal at apredetermined audio frequency. The base and collector of the transistor44 are inductively coupled by the coupling transformer 50 having aprimary winding 52 and a secondary winding 54 respectively connected tothe base and collector of the transistor 44. The transformer 50 is tunedfor sustained oscillation of collector current at the predeterminedaudio frequency. Collector bias is established by connection of thesecondary winding 54 in parallel with capacitor 56 to the positiveoutput terminal of the DC voltage source 22. Base bias on the other handis establ shed through the primary winding 52 connected in series withthe bias resistor 58 to the output voltage terminal of battery 22. Inorder to prevent any AC components from being conducted through thebattery 22, the primary winding 52 is connected to the grounded by-passcapacitor 60 and grounded bleed resistor 62. It will be apparenttherefore, that upon closing of the switch 24, operation of the audiooscillator 42 will begin immediately simultaneously with the radiofrequency oscillator 12.

The transmitter 10 includes a second audio oscillator 64 which issimilar in circuit arrangement to the audio oscillator 42. Thus, theaudio oscillator 64 includes a transistor 66 of the NPN type having anemitter connected to the grounded emitter resistor 68 and an outputcollector connected to the coupling capacitor 34 through the resistor70. The base and collector of transistor 66 are coupled by the tunedtransformer 72 having a primary winding 74 and a secondary winding 76.The secondary Winding 76 is connected in parallel with the capacitor 78to the output voltage terminal of battery 22 while the base oftransistor 66 is supplied with base bias from the voltage source throughthe bias resistor 80 in series with the primary winding 74 of thetransformer. The primary winding 74 is furthermore connected to a timingcircuit 82 which includes the grounded storage capacitor 84 having arelatively large capacitance value and a bleed resistor 86 connected inparallel with the capacitor to the primary winding. A tuning switch 88is connected in shunt relation to the capictor 84 and resistor 86 inorder to disable oscillator 64. The capacitance of capacitor 84 and theresistance value of bleed resistor 86 therefore determine the chargingperiod of the timing circuit during which operation of the audooscillator 64 is delayed upon closing of the switch 24. Thus, uponclosing of the switch 24, insufficient forward bias will be applied tothe base of transistor 66 until such time as the capacitor 84 is chargedto a value corresponding to the forward bias potential of the transistor66 causing it to conduct. When conducting, the audio oscillator 64 willproduce an oscillating output at the collector of transistor 66 of anaudio frequency different from the audio frequency associated with theaudio oscillator 42.

When the audio oscillator 64 is operating, a negative voltage componentis fed through the coupling capacitor 90 and diode rectifier 92 to thebase of transistor 44 asassociated with the audio oscillator 42. Thecapacitor 90 is also connected to the bleed resistor 94 in order tocontrol discharge thereof. The negative voltage conducted through diode92 applies a reverse bias to the base of transistor 44 producingcut-oif. Thus, as soon as audio oscillator 64 begins to operate, audiooscillator 42 stops operating.

It will be apparent from the foregoing description, that upon closing ofthe switch 24 radio frequency oscillator 12 and audio frequencyoscillator 42 will begin to simultaneously operate producing a carrierfrequency output modulated at one audio frequency by the audiooscillator 42. After a predetermined period determined by thecapacitance of capacitor 84 and the resistance of resistor 86, audiooscillator 64 begins to operate cutting off operation of the audiooscillator 42. Thus, the carrier frequency output of the radio frequencyoscillator 12 is sequentially modulated at two different audiofrequencies, modulation at the initial audio frequency persisting for arelatively short period of time after which it is followed by modulationat the audio frequency of the oscillator 64 for as long as the switch 24is held closed. The audio modulating frequency output of oscillator 64may of course be adjusted through the transformer 72. In order to adjustthe audio frequency output of oscillator 42, tuning switch 88 is closedin order to disable the audio oscillator 64 by grounding the base oftransistor 66. Thus, oscillator 42 will continue to operate withoutinterruption so that its output frequency may be adjusted by tuning thetransformer 72.

FIG. 2 illustrates a transmitter which is similar in circuit arrangementto the transmitter 10 previously described and is also somewhat similarin operation. The transmitter 110 thus includes a radio frequencyoscillator 112 having a transistor 114 of the NPN type, an inductiveloop 116 connected to the collector of the transistor, a tuningcapacitor 118 connected in parallel with the inductive output loop 116,a feedback capacitor 120 connecting the output circuit to the base oftransistor 114, a DC battery source of voltage 122 supplying DC voltageto the base of transistor 114 upon closing of the switch 124 through theresistor 126. Base bias for transistor 114 is maintained by the leakageresistor connected to the base through the input juncture 132 to whichthe modulating signals are applied through resistor 128 and couplingcapacitor 134. The radio frequency oscillator 112 also includes thechoke coils 136 isolating the battery 122 from the AC components of thecurrent conducted through the output circuit and the choke coil 138between the emitter of transistor 114 and the emitter resistor 140.

The transmitter 110 also includes an audio oscillator 142 having atransistor 144 of the NPN type, an emitter resistor 146, a couplingresistor 148 connecting the collector of the transistor 144 to thecoupling capacitor 134, a coupling transformer 150 having a primary 152and a secondary 154, the secondary being connected in parallel with thecapacitor 156 between the collector and the positive output terminal ofthe battery 122, the output terminal of the battery being connectedthrough the bias resistor 158 to the base in series with the primary152. Also, by-pass capacitor 160 and bleed resistor 162 are connected tothe primary 152. Thus, oscillator 142 is similar in arrangement andoperation to the oscillator 42 associated with transmitter 110. However,unlike the operation of the transmitter 10, oscillator 142 oftransmitter 110 is not initially operative simultaneously with the radiofrequency oscillator 112 upon closing of the switch 124 as will behereafter explained.

The other audio oscillator 164 associated with transmitter 110 includesthe transistor 166 having an output collector connected by the resistor170 to the signal coupling capacitor 134 for modulating the carrierfrequency output of the radio frequency oscillator 112. The base andcollector of transistor 166 are coupled by the transformer 172 having aprimary 174 connected to the base and a secondary 176 connected to thecollector. The secondary is connected in parallel with capacitor 178 tothe positive output voltage terminal of battery 122, the output terminalalso being connected by the bias resistor 180 to the base throughprimary 174 to which the bypass capacitor 184 and bleed resistor 186 areconnected. When the oscillator 164 is operating, reverse bias is appliedto the base of transistor 144 to prevent operation of oscillator 142 andtoward this end, the collector of transistor 166 is coupled by capacitor190 and diode 192 to the base of transistor 144, a bleed resistor 194being connected to the capacitor 190. Further, a timing circuit 182 isassociated with oscillator 164 in the form of a grounded storagecapacitor 198 connected through resistor 196 to the emitter oftransistor 166. A bleed resistor 188 is also connected to the juncturebetween capacitor 198 and resistor 196. As soon as transistor 166 beginsto conduct, the capacitor 198 is charged at a rate determined by itscapacitance and the resistance of bleed resistor 188 so that when apredetermined charge is attained, a cut-olf bias will be established atthe emitter of transistor 166 to stop operation of the oscillator 164.

It will be apparent from the foregoing description of the transmitter110 that upon closing of switch 124, radio frequency oscillator 112 andaudio oscillator 164 begin to operate immediately in order to produce acarrier frequency output modulated by the output of the oscillator 164at one predetermined audio frequency. At this time, operation ofoscillator 142 is prevented by the reverse bias applied to the base ofits transistor 144 through the diode 192. After a predetermined periodestablished by the timing circuit 182, operation of oscillator 164 stopsand the reverse bias applied to transistor 144 is removed. Oscillator142 thus begins to operate in order to modulate the carrier frequencyoutput of oscillator 112 at another audio frequency. The oscillator 164remains inoperative until the capacitor 198 is discharged through thebleed resistor 188 in order to begin another operational cycle. Thus,the carrier frequency output of radio frequency oscillator 112 isalternately modulated at two different frequencies in a cyclic fashion.

FIG. 3 illustrates a transmitter 210 which has the same operationalcapability as the transmitter 110 but in addition thereto may produce acarrier frequency output eX- clusively modulated at either one of thetwo audio frequencies between which modulation alternates when thetransmitter 210 operates in the same manner as hereinbefore described inconnection with transmitter 110. Transmitter 210 includes a radiofrequency oscillator 212 similar to the oscillator 112 both inarrangement and operation. The oscillator 212 is therefore provided witha transistor 214 of the NPN type having a collector con nected to theinductive loop 218 through which oscillating current is conducted at acarrier frequency adjusted by the tuning capacitor 218. A feedbackcapacitor 220 connects the output loop 216 to the base of transistor 214to which the leakage resistor 230 is connected at the input juncture 232for maintaining proper base bias. A DC source of voltage in the form ofbattery 222 is connected to the output circuit through the choke coil236 while choke coil 238 connects the emitter of transistor 214 to theemitter resistor 240. Modulating signals are applied to the inputjuncture 232 through resistor 228. Further, in the case of transmitter210, the oscillating output current in the inductive loop 216 is fedthrough the coupling capacitor 306 to the base of transistor 304 in aradio frequency amplifier circuit 302. The choke coil 308 is connectedto the base of transistor 304 to isolate the power supply from ACcomponents. The emitter of transistor 304 is grounded while thecollector is connected to the output terminal of the battery 222 throughload resistor 310, the outpu terminal also being connected to a filtercapacitor 312. Thus, the oscillating signal fed through signal couplingcapacitor 306 from the inductive loop 216 is amplified at the collectorof transistor 304 and the signal is fed through capacitor 314 to thetransmitter antenna 316.

' The audio oscillators 242 and 264 are similar in arrangement andoperation to the oscillators 142 and 164 associated with the transmitter110. Thus, the oscillator 242 includes a transistor 244 of the NPN typehaving an emitter connected to the emitter resistor 246, an outputcollector connected by resistor 248 to the coupling resistor 228 inparallel with the output from the oscillator 264. Coupling transformer250 includes a primary 252 connected between the base of transistor 244and the bias resistor 258 while the secondary 254 is connected inparallel with capacitor 256 between the collector and the outputterminal of battery 222. By-pass capacitor 260 and bleed resistor 262are also connected to the primary 252. Similarly, oscillator 264includes the NPN type transistor 266 having an output collectorconnected by the resistor 270 to the coupling resistor 228 and acoupling transformer 272 having a primary 274 connected between the baseof transistor 266 and the bias resistor 280 while secondary 276 isconnected between the collector and the output terminal of battery 222in parallel with capacitor 278. A by-pass capacitor 284 and bleedresistor 286 are connected to the primary 274 while the output collectoris connected by the coupling capacitor 290, diode rectifier 292 andresistor 293 to the base of transistor 244 for applying reverse biasthereto when the oscillator 264 is operating. A bleed resistor 294 isalso connected to the capacitor 290 similar to the arrangementshereinbefore described in connection with transmitters 10 and 110. Theoscillator 264 is also provided with a timing circuit 282 capable ofperiodically timing the operational periods of the oscillator 264 foralternate modulation of the carrier frequency output at the twodifferent audio frequencies corresponding to the oscillators 264 and 242as hereinbefore described in connection with the transmitter 110.

The alternate, sequential modulation of the carrier frequency output oftransmitter 210 occurs when the switch element 326 of the mode selectingswitch assembly 224 is displaced from the non-operating positionillustrated to the other operating position. In its non-operatingposition, the switch 326 shunts the storage capacitor 298 and 'bleedresistor 288 associated with the timing circuit 282. On the other hand,when the switch 326 is in its operating position, the capacitor 298 andbleed resistor 288 are connected to ground through the switch 324 in itsillustrated position and are connected in series with the resistor 296to the emitter of transistor 266. Thus, oscillator 264 will begin tooperate immediately simultaneously with the radio frequency oscillator212 since the switch 326 in its operating position will also ground thenegative terminal of battery 222 through the switch 324. When capacitor298 is charged to a cut-off potential value determined by itscapacitance and the resistance of bleed resistor 288, operation of theoscillator 264 stops. Reverse bias applied to the base of transistor 244through diode 292 is then also removed so that oscillator 242 may beginto operate until capacitor 298 is discharged sufficiently to beginanother operational cycle.

Assuming the switches 324 and 326 are in the positions illustrated inFIG. 3, movement of the switch 322 of the mode selector switch assembly224, to its closed position grounds the negative terminal of battery222. With the capacitor 298 and bleed resistor 288 shunted by switch326, the emitter of transistor 266 is connected through resistor 296 andswitch 324 to ground so that oscillator 264 immediately begins tooperate simultaneously with the radio frequency oscillator 212 uponclosing of the switch 322. The oscillator 264 remains operating sincethe timing circuit 282 is disabled by the switch 326. Accordingly, thecarrier frequency output of the transmitter is exclusively modulated bythe oscillator 264. On the other hand, when switch 324 is displaced fromits non-operating position illustrated in FIG. 3 to its other operatingposition, ground is removed from the emitter of transistor 266 at thesame time that ground is applied to the negative terminal of battery 222through ground line 320. Thus, operation of oscillator 264 is preventedso that the carrier frequency output of oscillator 212 will beexclusively modulated by the oscillator 242. It will be apparent fromthe foregoing, that the switches of the mode selector switch assembly224 will produce three modes of operation and that each operational modemay correspond to a different function labelled up, stop and down inFIG. 3.

As shown in FIGS. 4 and 5, the modulating component of the signaltransmitted by transformer 210 and derived from the audio oscillator 264is depicted by wave form 326 while the audio signal component derivedfrom oscillator 242 is depicted by wave form 328. These audio componentsof the signal as well as the carrier frequency of the transmitter 210are recognized by the receiver component 410 illustrated in FIG. 6. Asshown, the radiated signal is picked up by the antenna 412 and amplifiedin the radio frequency amplifier component 414 from which the signal isfed to a superregenerative type of detector component 416 tuned to thecarrier frequency of the transmitter 210. The audio frequency componentsare fed from the detector component 416 to the dual audio amplifiercomponent 418 from which these audio frequency signal components are fedto the frequency tuned networks 420 and 422. The network 420 for examplewill pass signal components in one narrow audio frequency band such asthe signal component 326 while the other network 422 will pass signalcomponents within a narrow audio frequency band including the frequencyof the wave form 328. In this fashion, the receiver component 410 willrespond only to the combination of carrier frequency and two audiomodulating frequencies associated with the transmitter 210.

The frequency tuned networks 420 and 422 establish two audio frequencysignal channels connected to a utilization circuit 424 through which thediverse functions are performed. Accordingly, there are three controldevices associated with the utilization circuit performing these diversefunctions consisting of the relay coil 426 performing an up function byclosing of the relay switch 428 upon energization thereof, a relay coil430 performing a stop function upon opening of the relay switch 432 whenthe relay coil is energized, and a relay coil 434 energized to close therelay switch 436 in order to perform a down function. The relay coil 426is energized in response to alternate modulation of the carrierfrequency signal by the two audio modulating signals whereas the relaycoils 430 and 434 are respectively energized by exclusive modulation ofthe carrier frequency output by one of the audio modulating frequencysignals. The audio frequency signals are operative in this fashion onthe utilization circuit 424 through the signal channels established bythe frequency tuned networks 420 and 422.

The network 420 is connected through resistors 438 and 440 to the baseof transistor 442 while the frequency tuned network 422 is connected bythe resistor 444 to the base of transistor 446. Grounded capacitors 448and 450 are respectively connected to the bases of transistors 442 and446 so as to delay switching of these transistors to the conductivestates when signals are applied to the bases from the signal channelsestablished by the frequency tuned networks 420 and 422. The transistors422 and 446 are connected in series to each other and toward this end,the emitter of transistor 442 is connected to the collector oftransistor 446. The collector of tran sistor 442 is connected to asuitable positive voltage source 452 While the emitter of transistor 446is connected through coupling resistor 454 to the input base oftransistor 456 having an emitter resistor 458 and an output collectorconnected by line 460 to one terminal of the relay coil 426, the otherterminal of the relay coil being connected to the positive voltagesource 452. Because of the rapidity with which the carrier frequencysignal is alternately modulated at the two different audio frequencies,in one mode of operation of the transmitter 210, and in view of thecapacitance of capacitor 448 relative to capacitor 450, transistor 442will remain conductive after the signal applied to its base from thenetwork 420 has ceased at the same time that the transistor 446 isswitched on by a signal applied to its base from the network 422. Thus,while transistors 442 and 446 are simultaneously conductive, a currentpath is established from the voltage source 452 to the input oftransistor 456 causing it to switch on. Since the base of transistor 456is connected to the capacitor 462 and bleed resistor 464, it remainsconductive for a sufficient length of time to energize relay coil 426and close the relay switch 428 in order to perform the up function. Atthe same time that the relay switch 428 is closed, the normally closedrelay switch 466 is opened so as to disconnect the voltage source 452from the relay coils 430 and 434 thereby preventing energization ofthese relay coils.

On the other hand, receipt of a carrier frequency signal modulatedexclusively by one of the audio modulating signals, will produce asignal in only one of the channels associated with the networks 420 and422. For example, the existence of only a signal from the network 420will apply forward bias to transistor 468 having an output collectorconnected to the relay coil 430 and an emitter connected to the emitterresistor 470. Storage capacitor 472 connected to the base of transistor468 delays switching on of the transistor 468 for a period sufficient topermit prior energization of relay coil 426 if there is any alternatemodulation of the carrier frequency signal. Similarly, an exclusiveaudio modulating signal will be fed from the network 422 throughresistor 474 to the base of transistor 476 causing it to switch on aftera delay determined by capacitor 478 in order to energize the relay coil434. Thus, the utilization circuit 424 will respond to the differentoperational modes of the transmitter 210 in order to perform the diversefunctions by exclusive energization of either relay coil 426, relay coil430 or relay coil 34.

The foregoing is considered as illustrative only of the principles ofthe invention. Further, since numerous modifications and changes willreadily occur to those skilled in the art, it is not desired to limitthe invention to the exact constructon and operation shown anddescribed, and accordingly all suitable modifications and equivalentsmay be resorted to, falling within the scope of the invention asclaimed.

What is claimed as new is as follows:

1. In a transmitter having a radio frequency oscillator, means formodulating the output of the radio fre quency oscillator comprising apair of audio oscillators connected in parallel to said radio frequencyoscillator having dilferent output frequencies, a source of voltage,each of said audio oscillators including a current conducting devicehaving a control electrode and an output electrode operatively coupledto the radio frequency oscillator, switch means for simultaneouslyconnecting the source of voltage to the control electrodes of thecurrent conducting devices to energize the audio oscillators, cut-offmeans coupling the output electrode of one of the audio oscillators tothe control electrode of the other of the audio oscillators forpreventing operation of said other of the audio oscillators when saidone of the audio oscillators is operating and timing means connected toone of the current conducting devices for temporarily preventingoperation of the' audio oscillator associated therewith.

2. The combination of claim 1 wherein said cut-01f means comprises acoupling capacitor connected to the output electrode of the currentconducting device associated with said one of the audio oscillators anda diode connecting the coupling capacitor to the control electrode ofthe current conducting device associated with the other of the audioosillators.

3. The combination of claim 2 wherein each of said audio oscillatorsfurther includes inductive coupling means interconnecting the controland output electrodes, said source of voltage being connected by theswitch means to the control electrode through the inductive couplingmeans.

4. The combination of claim 3 wherein each of said current conductingdevices comprises a transistor having a base constituting said controlelectrode, a collector constituting said output electrode and anemitter.

5. The combination of claim 4 wherein said timing means includes astorage capacitor connected to the emitter of one of the transistors.

6. The combination of claim 5 including additional switch means forselectively rendering either one or the other of the audio oscillatorsoperative to the exclusion of the other of the audio oscillators.

7. The combination of claim 1 wherein each of said audio oscillatorsfurther includes inductive coupling means interconnecting the controland output electrodes, said source of voltage being connected by theswitch means to the control electrode through the inductive couplingmeans.

8. The combination of claim 7 wherein said timing means comprises acapacitive delay circuit connected to the control electrode of thecurrent conducting device associated with said one of the audiooscillators for delaying conduction therethrough in response to closingof the switch means.

9. The combination of claim 8 wherein said cut-off means comprises acoupling capacitor connected to the output electrode of the currentconducting device associated with said one of the audio oscillators anda diode connecting the coupling capacitor to the control electrode ofthe current conducting device associated with the other of the audiooscillators.

10. A transmitter comprising a radio frequency oscillator having anoutput element and an input element, a pair of audio frequencyoscillators connected to the input element for modulation of the carrierfrequency output of the radio frequency oscillator at two audiofrequencies, a source of voltage, cut-off means interconnecting saidaudio frequency oscillators for preventing simultaneous operation ofboth audio frequency oscillators, timing means rendered operative forcyclically limiting operation of one of the audio frequency oscillatorsto predetermined intervals, and switch means connecting said source ofvoltage to all ofthe oscillators for simultaneous operation of the radiofrequency oscillator and one or the other of the audio frequencyoscillators.

11. The combination of claim 10 wherein said switch means includes afirst switch device for energizing both of the audio frequencyoscillators, a second switch device connected to said timing means fordisabling said one of the audio frequency oscillators, and a thirdswitch device connected to the second switch device for rendering thetiming means operative, whereby said carrier frequency output ismodulated only at one of the two audio frequencies or sequentiallymodulated at said two audio frequencies.

12. In combination with the transmitter defined in claim 11, a receiverhaving detector means tuned to said carrier frequency, a pair of audiofrequency responsive networks for establishing two signal frequencychannels, and utilization circuit means connected to said channelshaving three control devices respectively actuated by signalstransmitted through one of said channels at one of the two audiofrequencies and sequentially through both of said channels at the twoaudio frequencies.

13. The combination of claim 12 wherein said utilization circuit meansincludes a pair of series-connected switching devices connected to oneof the control devices, means independently connecting said signalchannels to the switching devices for sequentially controlling theswitching devices, delay means responsive to a signal at one audiofrequency in one of the channels followed by a signal in the otherchannel at the other audio frequency for establishing a current paththrough both of the switching devices to energize said one of thecontrol devices, means responsive to energization of said one of thecontrol devices for preventing energization of the other of the controldevices, and means connecting said signal channels to the other of thecontrol devices for energization thereof in delayed response to signalsin one of the channels at one of the two audio frequencies.

14. In combination with a transmitter radiating signals at a carrierfrequency modulated at two different audio frequencies, a receivercomprising detector means tuned to said carrier frequency, a pair ofaudio frequency responsive networks for establishing two signalfrequency channels, and utilization circuit means connected to saidchannels having three control devices respectively actuated by signalstransmitted through one of said channels at one of the two audiofrequencies and sequentially through both of said channels at the twoaudio frequencies.

15. The combination of claim 14 wherein said utilization circuit meansincludes a pair of series-connected switching devices connected to oneof the control devices, means independently connecting said signalchannels to the switching devices for sequentially controlling theswitching devices, delay means responsive to a signal at one audiofrequency in one of the channels followed by a signal in the otherchannel at the other audio frequency for establishing a current paththrough both of the switching devices to energize said one of thecontrol devices, means responsive to energization of said one of thecontrol devices for preventing energization of the other of the controldevices, and means connecting said signal channels to the other of thecontrol devices for energization thereof in delayed response to signalsin one of the channels at one of the two audio frequencies.

16. In a transmitter, an RF oscillator including a transistor having abase and a collector, an oscillating output circuit connected to saidcollector, a feedback capacitor coupling said output circuit to thebase, a source of DC voltage connected to the base, a leakage resistorconnected to the base for establishing base bias sustaining oscillationsin the output circuit at a carrier frequency, a pair of audiooscillators connected to the base for modulating the carrier frequencyoscillations, each of said audio oscillators including a transistorhaving base, collector and emitter elements, an inductive circuitcoupling the base and collector elements having primary and secondarywindings respectively connected to the base and collector elements, andbias means connecting the source of voltage to the base element throughthe primary winding for sustaining oscillations in the inductive circuitat audio frequencies, unidirectional coupling means connecting thecollector element of one of the audio oscillators to the base element ofthe other of the audio oscillators for applying a reverse bias thereto,and timing means lll primary winding for delaying establishment of aforward bias on the base element.

18. The combination of claim 16 wherein said timing means comprises astorage capacitor connected to the emitter element for cyclicallylimiting conduction through the transistor to predetermined operationalperiods of the 10 audio oscillator.

References Cited UNITED STATES PATENTS 3,316,488 4/1967 Reynolds 325-1053,339,141 8/1967 Rothebuhler 32537 ROBERT L. GRIFFIN, Primary ExaminerK. W. WEINSTEIN, Assistant Examiner US. Cl. X.R.

